Field of the Invention
The present invention relates to a fixing device to fix an unfixed toner image onto a recording medium in an image forming apparatus, and an image forming apparatus using the fixing device. To be more specific, the invention relates to a fixing device configured to change strength of a pressing force between two nip members (for example, rollers) which form a fixing nip, and an image forming apparatus using the fixing device.
Related Art
Heretofore, some image forming apparatuses are configured to change strength of a pressing force between nip members which form a fixing nip. Depending on types of a recording medium to be used, a preferable pressing force at the fixing nip differs. In one example of an image forming apparatus, there set two levels of strength of the pressing force. The stronger pressing force is adopted for a plain paper and the weaker pressing force is adopted for special recording media including an envelope.
Patent Document 1 discloses a fixing device enabled to switch the pressing force as mentioned above. The fixing device of the Patent Document 1 is provided with two springs to realize the pressing force in two levels. One of the springs is configured to continuously apply an elastic force on a fixing nip and the other one is configured to be switched its action and inaction of the elastic force. When the elastic forces of both the springs are applied, the pressing force is defined strong, and when the elastic force of only one spring is applied, the pressing force is defined weak.